1. Field of the Invention
The present invention relates to display systems and particularly to single or dual-panel spatial light modulator sequential color systems.
2. Description of the Related Art
Sequential color display systems, like single-chip micromirror systems, temporally filter the illumination source into primary colors. These typically has been implemented using a spinning wheel that has dichroic filter segments along the outer diameter of the wheel 10, as illustrated in FIG. 1a. For single-panel architectures, the primary color filter set of red 11, green 12, and blue 13 (R-G-B) light is used in every video display frame. Optionally, as shown in FIG. 1b, two-panel optical architectures may use a color splitting prism or dichroic mirror 18 to provide red light (R) to a first modulator panel, and a color filter wheel 15 with yellow 16 (Y) and magenta 17 (M) filters to alternately provide blue (B) and green (G) light to a second modulator panel.
Sequential display systems sometimes add a white (clear) segment to the color wheel 20, as shown in FIG. 2a and disclosed in U.S. Pat. No. 5,233,385, to improve the sequential color efficiency by applying a gain function to the luminance portion of the signal and displaying some portion of the luminance signal during the white segment time, in order to provide a brighter picture on the display. In operation, the white energy (luminance) in each pixel is gained-up by a certain factor and if the R-G-B signal levels saturate, then energy is subtracted from the R-G-B channels and shifted to the white segment. In order to maximize brightness, the outputs of the R-G-B channels are also maximized for a full-white signal. As shown, the color wheel 20 consists of red (R) 21, green (G) 22, blue (B) 23, and clear or white (W) 24 segments.
FIG. 2b shows a color filter wheel 25 with a white segment used in a two-DMD color projector system. In this case, the color filter wheel 25 consists of yellow (Y) 27, magenta (M) 28, and white (W) 29 segments.
Sequential color systems exhibit an undesirable characteristic when eye motion occurs in localized area of black and white pixels in a given image. For relatively slow moving objects, leading edges appear to have a color hew to them, which corresponds to the first color in the color sequence while trailing edges appear to a have color hew of the last color in the color sequence. In scenes that induce rapid eye motion, a color rainbow effect is created that has the appearance of color ghost images in these black and white areas of the picture. In the past, this undesirable color separation has been addressed by means of faster sequencing of the colors; either by faster rotation of the color wheel or by splitting the color wheel filters into multiple sets of R-G-B segments. However, both of these approaches introduce negative factors, such as: (1) audible noise and less mechanical stability when operating the color wheel at higher speeds, (2) decreased efficiency (loss of brightness) due to additional color wheel spokes when adding additions filter segments, and (3) higher cost and (4) increased temporal artifacts (pulse width modulation noise).
There is a recognized need for a method which addresses the color separation problem discussed above in a more elegant way without introducing the new negative factors discussed immediately here above. The invention disclosed herein addresses this need in both a method and an apparatus.
This invention discloses the method and apparatus for reducing the color separation in a sequential color display system. The disclosed approach adds a white (clear) segment to the color filtering system and applying an algorithm, which separates the luminance channel as much as possible into this white segment. As a result, in the case of a black-and-white image where color separation is most visible, color segments will have a minimal amount of energy in them, thereby reducing the color separation effect.
The algorithm uses a constant (xcex1), defined as the ratio of the white segment time to the smallest color segment time, to control the process of transferring as much of the energy as possible from the colored segments into the white segment.
The disclosed method extends the process to dynamically adjust the color segment (Rgain, Ggain, and Bgain) values on a pixel-by-pixel basis, such that colored areas of the image are maximized for brightness and black-and-white areas are maximized for minimum color separation. This allows a trade-off to be made between maximum brightness for the color areas and minimum color separation for the black-and-white areas of the image.